Opening Te Waihora/Lake Ellesmere
Te Waihora/Lake Ellesmere Level Information
Te Waihora/Lake Ellesmere is currently CLOSED to the sea. The calm average level as at 8am on Monday 21 September was 0.84m.
Q: What was the lake level reading before this one?
A: 0.82m. Wind-affected average level as recorded at 8am on Monday 14 September 2020.
Q: When was the lake last opened to the sea?
A: The lake was mechanically opened on 16 July 2020 and closed naturally on 11 August 2020.
Q: At what levels can the lake be opened to the sea at this time of year?
A: The lake may be opened at any level between 15 September and 15 October.
The next update will be on 28 September.
24/7 access to Te Waihora/Lake Ellesmere water levels:
Te Waihora/Lake Ellesmere levels are now available on our river flow data web page. These levels are average readings taken from the two gauges around the lake, one at Taumutu and the other at Seabridge Road/Nutts Cut. Please note that wind effect should be taken into account when interpreting this raw data. The wind information is currently not provided on the public web page, however, it may be requested by contacting Customer Services.
Te Waihora/Lake Ellesmere is the largest lake in Canterbury and has no natural outlet to the sea. It is valued for cultural and environmental reasons and was opened by generations of Ngāi Tahu before Pākehā arrival. The first written settler's record of an artificial opening between the lake and sea was in 1852. It has been opened over 300 times since.
Te Waihora/Lake Ellesmere's opening is governed by a National Water Conservation Order and a range of resource consents held jointly by Te Rūnanga o Ngāi Tahu and Environment Canterbury. However, many groups have an opportunity to air their views before a final decision is made.
Openings tend to be aligned with fish migration periods and other habitat values of the lake. Tuna (freshwater eels) come in from April to June via kōumu/drains or a full opening. Pātiki/flounder, inānga/whitebait, sea-run trout and other species enter the lake around September and October.
Factors to take into account include:
- access for traditional mahinga kai including gathering of swan's eggs
- avoiding low lake levels in summer and desiccation of wetland margins
- the need for variation in lake levels to support complex and diverse wetland bird and plant habitats
- managing land inundation (including from wind effects)
- the effects of high lake levels on drain networks and infrastructure.
Experts provide technical advice around the machinery. The weather and sea conditions are forecast while lake levels are monitored. It generally takes three to seven days to forge the opening but if sea conditions are rough, past experience has shown it could take up to six weeks.
Historical methods for opening the lake have included horse-drawn scoops and two outlet culverts in the early part of the 20th century - Dobson’s Culvert and Pannett’s Culvert. Heavy machinery has been used since the 1930s. For more details, read these accounts.
Once the lake is opened, there is limited ability to control how long it stays that way. The length is determined by weather and sea conditions, and how quickly gravel is deposited by the sea to close the cut.
Artificial closure has not been attempted to date however the water order allows for the lake to be artificially closed when it reaches 0.6 metres above sea level between 1 October and 31 March. Such a closure would only be considered in summer if lake levels were very low and sea conditions were too calm to create natural closure.
We monitor effects of the lake opening including:
- keeping an eye on channels through the spit from the lake edge to assist tuna/freshwater eel migration
- sediment sampling and experimental dredging of the deeper channels of the lake bed to work out whether this will result in greater flushing to improve water quality and associated wildlife and mahinga kai values
- investigating the feasibility of opening the lake at the southern end of Te Korua near the mouth of the Waikekewai Creek and the conditions needed in the lake to make a successful opening in this area (Te Korua was the traditional opening site used by Ngāi Tahu)
- investigating the feasibility of establishing a controlled outlet from Te Waihora/Lake Ellesmere to the sea.
High lake levels can result in flooding of land surrounding the lake and reduce the efficiency of local drainage systems. Other drivers for opening the lake include recognising the value of the lake for mahinga kai, wildlife and wetland vegetation.
The lake may be opened to the sea whenever the lake level:
- exceeds 1.05 metres above mean sea level, from 1 August to 31 March; or
- exceeds 1.13 m above mean sea level, from 1 April to 31 July; or
- at any level, from 15 September to 15 October; or
- at any level, from 1 April to 15 June.
Closing of the lake may be required to prevent extended periods of low lake levels over summer. Low lake levels impact water quality, lake edge vegetation and ecology within the lake.
The physical constraints of working on the beach, the wave environment and the very low lake level mean that artificially closing the lake would be very difficult and may only be successful if the lake was closing naturally.
A consultation group made up of a number of organisations representing different interests, give advice on the potential opening. The consultation process is set out in the Resource Consent Consultation Protocol. The consultation group is:
- Te Taumutu Rūnanga
- The Department of Conservation
- Lake Ellesmere/Te Waihora Rating District Liaison Committee
- North Canterbury Fish and Game Council
- Lake Ellesmere Commercial Fishermen
- Selwyn District Council
- Christchurch City Council
- Waihora Ellesmere Trust (Key Advisor)
The channel connects to a deepwater pool within the lake that is widened at the lake end to maximise flow into the channel. This pool is protected by a seawall (a gravel bund) to prevent shingle infill. The difference in water levels between the lake and the sea is another vital factor for opening the lake.
The final cut to the sea is typically made a couple of hours after high tide to take advantage of the largest difference in water levels at low tide. The opening is then left to scour naturally.
The mouth of the channel may develop to be more than 100 metres wide and 3-5 metres deep.
In difficult weather and adverse sea conditions, multiple attempts may be necessary and a successful opening may take weeks or even months to achieve.
An opening is considered “successful” when it persists for at least four days (the amount of time required on average to lower the lake level below the opening threshold).
- diverting the lake outfall to the Rakaia River mouth
- diverting via the Huritini/Halswell River to the Ihutai/Avon-Heathcote Estuary
- diverting via a canal to Wairewa/Lake Forsyth
- a breakwater to protect the opening.
More information: http://www.hydroeco.net/tewaihora/